MT Campground Suggestions along I-90

This is a great place to stay.

Jim and Mary's RV Park - Missoula

There is a Cabelas in Billings--they usually have RV parking and a dump. There is a large area to stop overnight just North of the freeway in Missoula, Exit Reserve St (South of the freeway is a large RV company with services). A forest Service CG is just off the freeway at about MM 60 on I-90 (at the rest area) if you're going that far West.
O K, I'm cheap--but, if yer looking to just stop for the night, these will work.

Spring Creek campground in Big Timber is nice. There are also a couple of FREE Montana Fishing Access sites near there, close to I-90. No amenities but a vault toilet.... but location is everything!

x2 on Jim & Mary's in Missoula and
x2 on Billings KOA